The Osaka District Court has invalidated Nuclear Regulation Authority (NRA) approval for the No. 3 and 4 reactors to operate at Kansai Electric Power Co.'s Oi nuclear power station in Fukui Prefecture, on the Sea of Japan coast. The ruling is the first to revoke national approval for a reactor since the March 2011 Fukushima nuclear disaster.
The Oi plant case centered on the validity of the "design basis seismic ground motion" calculation, used to determine what earthquake-proofing a facility requires. The calculation predicts a reactor's expected maximum oscillation, and is based on an average of previous earthquake data.
However, the NRA's assessment guidelines call for not just that average, but for nuclear plant operators to consider the data dispersion -- the full range of earthquake intensities in the historical data -- when drawing up earthquake resistance measures. Kansai Electric did not include a "buffer" to reflect the possibility of temblor intensities exceeding the historical average in its plans, and the plaintiffs in the case claimed that the average value was too low.
In its ruling, the Osaka District Court noted that the NRA did not consider whether adding the buffer was necessary for the Oi plant, which was "erroneous and flawed in a way that should not be overlooked." The court concluded that the agency's May 2017 approval for the facility's No. 3 and 4 reactors was unreasonable and illegal.
The NRA drew up new nuclear plant safety standards for resuming reactor operations based on the experience of the triple-meltdown at the Fukushima Daiichi Nuclear Power Station in 2011. Then NRA head Shunichi Tanaka boasted that the new standards were the "toughest in the world."
The Osaka District Court, however, heaped criticism on the agency for failing to uphold its own standards.
The Oi station's No. 3 and 4 reactors are currently offline for regular inspections. The government agency is expected to appeal the Osaka court's decision, so the ruling will not take effect immediately.
However, the same standard ground motion calculation method used for the Oi plant has also been used for most reactors across Japan. If the Osaka court ruling is eventually finalized after being upheld in any appeal rulings, the NRA will inevitably be called upon to redo all of its plant assessments.
It is the NRA's responsibility to restore both the safety of and trust in Japan's nuclear power industry, harnessing the lessons of the Fukushima disaster. The recent Osaka District Court ruling has revealed that the agency's safety assessments have been insufficient, and the NRA should accept the court's decision with the gravity it deserves.
Since the Fukushima nuclear disaster, multiple courts have issued orders to halt nuclear reactors. None have ever been finalized and gone into effect. However, it is incumbent on the NRA not to forget the lessons of the past, and to continue to strive to improve nuclear safety. If it does not, nuclear power will never regain the trust of the Japanese people.